The present invention relates to offset control circuits for adjusting an output voltage so as to correspond to an input voltage.
Conventionally, an offset control circuit as shown in FIG. 16 is used for adjusting the amount of offset contained in the input and output of an operational amplifier, for example.
In FIG. 16, an offset control circuit 200 is configured by connecting the output terminal of an operational amplifier 201 (op-amp) in series with resistors R1 and R2, as well as a variable voltage generating source 202, and connecting the point where the resistors R1 and R2 are connected to the negative input terminal of the operational amplifier 201. An input voltage VIN+ is input to the positive input terminal of the operational amplifier 201, and an output voltage VO is output from the output terminal of the operational amplifier 201.
Here, in order to apply an offset voltage to the input voltage VIN of the operational amplifier 201 and generate the output voltage VO, an output voltage V2 from the variable voltage generating source 202 is freely changeable. Due to the characteristics of the operational amplifier 201, an input voltage V1 (VIN−) applied to the negative input terminal of this operational amplifier 201 is the same as the input voltage VIN+ to the positive input terminal. The output voltage signal VO from the operational amplifier 201 is determined by the resistors R1 and R2, the input voltage VIN+ and the output voltage V2. For this reason, the output voltage VO can be output from the operational amplifier 201 while adjusting its offset with respect to the input voltage VIN by adjusting the output voltage V2 with the variable voltage generating source 202.
The above-described conventional offset control circuit 200, however, is not suited to high-speed operations, because it uses the operational amplifier 201, which operates at low speed. Moreover, the operational amplifier 201 in general cannot operate normally without having a band that is 10 to 100 times the signal band used, so that it is necessary to use powerful transistors, and it is necessary to use even more powerful transistors, in order to configure the operational amplifier 201 so as to be capable of operating at high speeds. This results in the problem of significantly increased power consumption and an increased scale of the circuit.
Further, the above-described conventional offset control circuit 200 is configured such that a single input voltage VIN+ is input to the operational amplifier 201. Therefore, if the input voltage signal VIN+ contains any undesirable components such as noise components, the distortion characteristics of the signal voltage deteriorate, reducing the quality of the output voltage VO.
In order to prevent such deterioration of the distortion characteristics, it is conceivable to input a pair of differential input voltages to the offset control circuit to generate a differential output voltage proportional to the voltage difference between the two input voltages, and use this for the offset adjustment. When a highly accurate analog signal processing is actually performed, it is necessary to carry out the offset adjustment with the differential output voltage in order to prevent the deterioration of the distortion characteristics of the signal voltage.
However, in order to carry out the offset adjustment with the differential output voltage by using the above-described conventional offset control circuit 200, it is necessary to use two offset control circuits 200, resulting in the problem of an increased scale of the circuit.